This invention relates to an electronic meter for measuring functions, such as power and energy, of an alternating voltage and current to an electrical load.
Electronic kilowatthour meters have the potential to provide more accurate measurement of energy than the electro-mechanical rotating disk type of meter which is currently in use. However, for an electronic meter to supplant the electromechanical meter, it must be a highly reliable and economical device as well. As variety of circuits have been conceived in pursuit of these objectives.
One of the general approaches to the problem is to integrate the product of the instantaneous value of the voltage and the instantaneous value of the current. Another approach takes into account the basically sinusoidal waveforms of the voltage and current, and computes power as the product of the peak amplitude of the voltage, the peak amplitude of the current and the power factor. The present invention is an approach which generally falls into the latter category.
If energy is to be computed by multiplying peak voltage, peak current and power factor, it is desirable to avoid having to make a separate determination of the phase angle or power factor. One solution to this problem is found in the U.S. Pat. No. 4,056,774 to Shum. In that patent, a meter samples the voltage at its peak and also samples the current at the peak of the voltage waveform. It turns out that the current thus sampled is equal to the peak current amplitude times the power factor. This value can then be multiplied by the voltage to compute power and energy.
In the present invention, it is recognized that (1) the integral of a half cycle of the voltage is proportional to is peak amplitude and (2) the integral of the current between zero crossings of the voltage waveform is directly proportional to the peak amplitude of the current multiplied by the power factor. Accordingly, such integrals can be used to compute power or energy. Circuit means have been conceived which allow a microprocessor to derive the integral values from an analog integrator in order to perform the necessary power and energy computations.